1. Field of the Invention
The present invention relates to a semiconductor chip with bumps, and more particularly to a semiconductor chip having a large solder ball wetting area so as to form the bumps of which the height is lower.
2. Description of the Related Art
As electronic devices have become smaller and thinner, the speed and the complexity of IC chip become more and more higher. Accordingly, a need has arisen for higher packaging efficiency. Demand for miniaturization is the primary catalyst driving the usage of advanced packages such as chip scale packages (CSP) and flip chips. Both of them greatly increase the packaging efficiency and reduce the amount of board real estate required when compared with the alternative ball grid array (BGA) and thin small outline package (TSOP).
The flip chip package itself also has to be miniaturized in dimensions. Specifically, not only the length and the width but also the thickness of the flip chip package has to be reduced. Referring to FIGS. 1 and 2, they depict the ball mounting process of a conventional semiconductor flip chip 18. The semiconductor chip 18 has a plurality of pads, which are typically aluminum pads 20. A passivation layer 16 is coated on the active surface of the semiconductor chip 18 with the aluminum pads 20 exposed. A repassivation layer 14, for example, which is made of BCB (benzocyclobutene), is coated on the passivation layer 16 with the aluminum pads 20 exposed. A plurality of UBMs (Under Bump Metallurgy) 22 are disposed on the exposed aluminum pads 20 for defining a plurality of bump pads. A plurality of preformed solder balls 12 are placed on the UBMs 22, i.e. the bump pads, and then reflowed to form a plurality of bumps 13. The process which uses the preformed solder balls 12 placed on the UBM 22 and reflowed to form the bumps 13 is generally referred to as a ball mounting process or a ball mount process.
Furthermore, it will be apparent to those skilled in the art that a redistribution process for the semiconductor flip chip can be carried out if the aluminum pads of the semiconductor flip chip are not positioned in place. In this case, the UBMs are formed on the repassivation layer and are electrically connected to the aluminum pads for forming a plurality of redistribution traces and a plurality of bump pads at other positions. Generally, another repassivation layer, for example, made of BCB (benzocyclobutene), is provided to cover the redistribution traces with the bump pads exposed.
In the prior art, the diameter w of the opening of the bump pads is about 80% to about 90% of the diameter d of the preformed solder balls, and the height h of the formed bumps is about 80% to about 85% of the diameter d of the preformed solder balls. If the height of the bumps has to be reduced, the small preformed solder balls will be required. However, as the size of the solder ball is reduced, the ball mounting process becomes more difficult and the cost also be increased.
Furthermore, in the prior art, the bump of which the height is less than 160 μm is generally formed by a standard flip chip process, including printing or plating solder paste and reflowing the solder paste. The bump which is made by the ball mounting process is generally larger than 240 μm in height. In fact, in the prior art, there is no suitable processes for forming the bumps which is 160 μm to 240 μm in height.
Accordingly, there exists a need for a semiconductor flip chip with the bumps which are made by a ball mounting process and are low in height.